Glass bottles, particularly those used for soft drinks and beer, are frequently reused. In order to reuse these, the bottle must be cleaned and sterilized once returned to the bottler. This is accomplished using a bottle washing apparatus which typically has three to five sections. The bottles are introduced to the machine and supported throughout the washing process by individual holders or pockets. Bottles are initially directed to a pre-rinse section which is designed to remove large particles and labels. In this section, water and residual chemicals from the cleaning process are directed at the bottles as they are introduced into the machine.
Next the bottles are conveyed into one or more cleaning or soaking sections where they are soaked in a highly caustic solution at an elevated temperature, generally about 70.degree. C. or higher. Pressurized spraying of the wash solution onto and into the bottles may also occur in these section(s). After 7 to 15 minutes, the bottles are taken to a warm rinse, and then to a final potable water rinse. The final rinse water is then re-used for the subsequent initial pre-rinse.
The cleaning or soaking solution is a highly caustic solution. Generally, this contains 2-4% sodium hydroxide. This acts to clean the bottles, dissolve metals such as metal foils contained in the main label and the band label on the neck of the container. Also, in combination with temperature and contact time, this highly caustic solution renders the bottles commercially sterile.
Although some operators clean bottles with caustic by itself, this is generally unacceptable. Caustic alone is not an effective detergent and it is not free rinsing. Further, it will cause calcium and magnesium ions to precipitate out of solution in the form of salts. The resulting precipitate leaves spots on bottles and can accumulate on the equipment as a tenacious scale. Such scale reduces the efficiency of the bottlewasher and increases energy consumption by hindering heat transfer.
To improve rinsability and detergency, a surfactant(s) is generally added to the soaking solution. The surfactant(s) serves many purposes. It helps the caustic wet and penetrate the soil and aids in soil dispersion. Surfactants also prevent soil from redepositing back onto clean bottles, provide foam control and enhance overall cleaning.
To prevent calcium and magnesium precipitation, a combination of chemicals is used. Chelants are used, which bind to the cations and prevent them from reacting with other detergent components and precipitating out of solution. Typical chelants include ethylene diamine tetraacetic acid, nitrilotriacetic acid, complex phosphates and alkali metal salts thereof, as well as the alkali metal salts of gluconic acid.
Sequestrants are also used to prevent hardness precipitation. The primary sequestrants are the orthophosphates and phosphonates. Scale is also controlled through the use of crystal growth inhibitors. These are the polycarboxylic acids such as polyacrylic acid, polymethacrylic acid, polymaleic acid, and copolymers thereof. These also act as threshold inhibitors and play a very important role in the overall process. Residual polycarboxylic acid is carried from the warm rinse back over to the pre-rinse. Thus, it prevents scale formation in the pre-rinse section, as well as scale formation and spotting in the rinse sections.
The caustic bottlewash process is not only chemically aggressive on the glass but caustic solutions also severely deteriorate permanent labels conventionally used on bottles. Permanent labels are generally colored with heavy metal pigments and glass lead based frit. These labels, known in the industry as applied color labels or applied ceramic labels (ACLs) are vulnerable to attack by the caustic solutions which in turn leach the heavy metal pigments from the labels into the wash solution.
The attacked labels appear faded, blackened, lackluster and bled. This deterioration of the labels is undesirable for beverage and brewery bottle washers because of the negative impact it has on brand image, consumer appeal and quality of the beverage package. Conventionally, the bottles are washed up to about 50-100 wash trips before the bottles are discarded or lost in trade. When the applied color labels deteriorate during these trips the bottles must be discarded before the end of their useful lives. Some bottle washers/manufacturers elect to use paper labels rather than the ACLs to extend the lives of the reused bottles. Additionally, it is possible for the leached metal pigments to redeposit onto or into the glass bottles posing a risk for contamination of consumable beverages.